1. Field of the invention
The present invention relates to processing seismic data, in particular to processing multi-component seismic data. In particular, it relates to processing multi-component seismic data to determine an event in one component that corresponds to an event in another seismic data component.
2. Description of Related Art
FIG. 1 is a schematic illustration of a seismic survey. As is well known, a seismic survey is performed using at least one seismic source 1 and an array of seismic receivers 2, 3, 4. In FIG. 1 the source 1 and the receivers 2, 3, 4 are shown disposed on the earth's surface, but other arrangements are known such as, for example, disposing receivers in a borehole. When the source 1 is actuated, acoustic energy is emitted downwards into the earth, and is reflected by geological structures within the earth. The reflected energy is detected at the receivers 2, 3, 4.
FIG. 1 shows two geological structures 5, 6 that act as partial reflectors of acoustic energy. These structures may, for example, be interfaces between two layers of the earth's interior that have different acoustic properties. As a result, the data acquired at each of the receivers 2, 3, 4 contains one “event” corresponding to partial reflection of acoustic energy at the upper interface 5 and another “event” corresponding to partial reflection of acoustic energy at the lower interface 6. When seismic data are processed, each interface responsible for reflection of seismic data is allotted a unique “interface index”, and events may be classified according to the index of the interface responsible for the event.
Typical traces that would be acquired by the receivers 2, 3, 4 of FIG. 1 are shown in FIG. 2. The vertical axis of FIG. 2 denotes time since the actuation of the seismic source. The traces are arranged in order of increasing source-receiver distance (known as offset). Thus, the trace having the lowest offset is at the left, and offset increases to the right. Within each trace, the horizontal scale provides a measure of the amplitude of seismic energy acquired at each receiver. “A2”, “A3” and “A4” denote the amplitude of the trace acquired at the receiver 2, at the receiver 3 and at the receiver 4 respectively.
If the three traces in FIG. 2 are compared, it will be seen that each trace contains an event B corresponding to reflection at the upper interface 5 and another event C corresponding to reflection of acoustic energy at the lower interface 6. However, events corresponding to reflection at a particular interface do not occur at the same time in each trace. For example, the event B occurs at time t1 in the trace acquired at receiver 2, but occurs at greater times in the traces acquired by other receivers. This is because the overall path length from the source to the receiver, and hence the travel time, increase with increasing source-receiver separation. The increase in arrival time of an event with increasing offset is known as “moveout”.
Many seismic surveys use multi-component seismic receivers, which are able to acquire at least two components of the seismic wave field incident on the receiver. A 3-component, or 3-C, receiver, for example, records three orthogonal components of the seismic wavefield, and these are normally taken to be the x-, y- and z- (vertical) components of the wavefield.
Acoustic energy emitted by the seismic source 1 is predominantly a pressure-wave (or p-wave). When the energy undergoes reflection an interface 5, 6, however, it may also undergo partial mode conversion to a shear wave (s-wave). The seismic wavefield acquired at the receiver 2, 3, 4 will therefore both contain p-waves and s-waves. Events arising from arrival of p-waves are generally referred to as PP events, since they involve seismic energy that is emitted as a p-wave and that is incident on the receiver as a p-wave. Events arising from s-waves are generally referred to as PS events, since they arise from acoustic energy which is emitted as a p-wave and which undergoes mode-conversion to an s-wave upon reflection and so is incident on the receiver as an s-wave. PP events occur most strongly in vertical components of the acquired seismic data, whereas PS events appear most strongly in the horizontal component of the acquired seismic data
Where partial mode conversion occurs, the seismic energy reflected as a p-wave gives rise to a PP event in the acquired seismic data and the seismic energy reflected as a (mode-converted) s-wave gives rise to a corresponding PS event in the acquired seismic data. A PP event and a PS event are said to be “corresponding events” if the PP event and the PS event involve reflection at the same interface within the earth's interior. The velocity of propagation of a p-wave through the earth is generally not equal to the velocity of propagation of an s-wave, so that a PP event in seismic data acquired at a receiver will in general not occur at the same time as the corresponding PS event. When multi-component seismic data is processed, it is often desirable to identify corresponding pairs of a PP event in the vertical component of the seismic data and a PS event in a horizontal component of the seismic data. This allows information about the reflector to be obtained from the PP data and from the PS data
The data traces shown in FIG. 2 represent seismic data traces essentially as acquired at the receiver 2, 3, 4. These are generally referred to as “raw” data traces.
In conventional seismic data processing, the raw data traces of FIG. 2 are first processed to compensate for the source-receiver offset. The effect of this processing is to transfer each event in a trace to the time at which it would have occurred if there had been zero source-receiver offset—i.e. if the source and receiver were co-incident. If the correction for offset is performed correctly, an event corresponding to reflection at one interface should occur at the same time in each offset-corrected trace—the event should have zero moveout in the corrected traces, and correction to zero-offset is therefore know as “moveout correction”. The traces corrected to zero offset may then be averaged, or “stacked”, and this attenuates random noise in the traces.
Methods have been proposed for identifying corresponding PP and PS events in stacked seismic data. In general, these methods assume that there is a constant linear relation between the arrival time of a PP event and the arrival time of the corresponding PS event. The arrival time of an event in the PP data can be mapped onto the expected arrival time of the corresponding event in the PS data by multiplying the PP arrival time by a constant factor, known generally as “vertical gamma”. The “vertical gamma” factor is essentially a squeeze/stretch factor, that stretches or squeezes the vertical axis (time axis) of traces for a vertical component of the seismic data to have the same scale as the vertical axis (time axis) of traces of a horizontal component of the seismic data.
The magnitude of the “vertical gamma” factor may be determined simply by manual identification of pairs of corresponding PP and PS events in the stacked seismic data, and deriving the vertical gamma factor from their respective arrival times. It is also known to use an interactive approach in which an initial value of the vertical gamma factor is picked from the stacked traces for the horizontal and vertical components, and is then used to assist in identification of further pairs of corresponding PP and PS events. Once further pairs of corresponding events have been identified, their arrival times may be used to refine the value of the vertical gamma factor.
These prior art techniques may not, however, be applied to raw data traces of the type shown in FIG. 2. The source-receiver offset varies from one raw trace to another, so that the arrival time of an event depends on the offset as well as on the velocity of propagation of acoustic energy. It is therefore not possible to match the arrival times of events in raw horizontal data traces with arrival times of events in raw vertical data traces using a constant scaling factor.